1. Field of the Invention
This invention relates in general to signal processing devices in telecommunication systems, and more particularly to a common mode bias voltage generator apparatus and method used in signal processing devices.
2. Description of Related Art
In many signal processing devices, such as a switched capacitor circuit, it is often necessary to generate bias voltages proportional to a supply voltage VCC or VDD. For example, to maximize a dynamic range of a differential amplifier, it may be desirable to generate a VCC/2 (or VDD/2) bias voltage to use as a common mode output reference.
Common mode bias voltages can be generated with many circuits. One circuit to generate a common mode bias voltage is a capacitively bypassed resistor divider. However, a simple resistor divider may not provide the best trade off of power dissipation and circuit area to meet the output impedance, settling time, and/or noise performance required for an intended or required use of a common mode bias voltage generator.
A simple resistor divider generally includes a couple of resistors serially connected to each other. To provide required power output, the output impedance of a resistor divider is often much higher, thereby significantly affects the settling time and noise performance of the entire system. To reduce the output impedance, a simple resistor divider is often buffered with a full-blown power amplifier to obtain required output power. This type of bias voltage generator may require an additional off-chip power amplifier. If a power amplifier is built on-chip, it would increase the size of the chip design and may be difficult to design in high speed applications. Further, this type of bias voltage generator is not the best trade, off of power dissipation and circuit area to meet the output impedance, settling time, and/or noise performance, etc.
In a switched capacitor circuit, a transient switch is often modeled as a resistor with a particular value. To obtain a better settling time and/or noise performance, it is generally desired to have a common mode bias voltage proportional to a supply voltage with a lower output impedance while using relatively little power and circuit area.
It is with respect to these and other considerations that the present invention has been made.